From DocWiki

Trust External Authentication represents a broad-range of enterprise security software that provides functions like authentication, resource access authorization, Single Sign On (SSO), and intrusion detection. Typically, this software protects your web server by installing a DLL plug-in into the web server service, for example IIS. This DLL plug-in, also called ISAPI Filter, intercepts user login credentials and passes them to a corporate authentication and authorization server. The software must be able to output user IDs in the HTTP header so that they can be passed to Cisco Unified MeetingPlace for authentication.

Note: User IDs in the Cisco Unified MeetingPlace profile database are case sensitive. Users cannot log in to Cisco Unified MeetingPlace as guests after you have configured this authentication mode.

Restrictions

When configuring Trust External authentication, make sure that the /mpweb/scripts/public/ directory is not protected by SSO. Protecting this directory will prevent Cisco Unified MeetingPlace Web Conferencing from functioning properly.